1. Field of the Invention
The purification of glycerol obtained from naturally occurring fats and oils by an improved distillation process.
2. Statement of the Relevant Art
Glycerol (propane-1,2,3-triol) is the most important of the trihydric alcohols. It is present as its esters in all animal or vegetable fats and oils and is obtained from them as a by-product of the saponification process during soap manufacture. Although glycerol is also synthesized from petrochemicals, this invention is concerned only with the purification of glycerol obtained from naturally occurring fats and oils.
Glycerol is formed during the transesterification, splitting or saponification of natural oils and fats, by saponification with alkali or by high pressure hydrolysis, and--depending on the saponification process--contains relatively large quantities of water, inorganic salts, fats, low molecular weight organic compounds and also higher glycerol oligomers and polymers.
The organic impurities accumulating in the crude glycerol consist primarily of fats and glycerol-like compounds formed by bacterial or chemical decomposition, such as propane-1,2-diol and propane-1,3-diol or even other components, such as glycerol methyl ethers which barely differ from glycerol in their physical properties (boiling point, refractive index) and which interfere seriously with the purification process because they are distilled together with glycerol in conventional columns having a small number of theoretical plates. Some of these impurities are responsible for the undesirable discoloration and poor color stability of the crude glycerol; accordingly, their separation is absolutely essential.
One feature common to all processes carried out on an industrial scale for extracting purified glycerol obtained from natural fats and oils is that production of high purity glycerol requires a crude mixture of which the organic impurity content (mong content) amounts to .ltoreq.1% by weight and from which fat, soap and other organic constituents have been removed. To this end, the fats are saponified with calcium hydroxide solution or by the lime-soda (calcium-sodium hydroxide) process in an elaborate chemical prepurification stage and are filtered off as far as possible in the form of soaps. Further separation, particularly to remove inorganic impurities, is carried out by distillation. Columns in which the plates produce a high pressure loss are used in the rectification stage. In order to obtain adequate separation of the various components, the sump temperatures have to be correspondingly high. Limits are imposed on this process parameter by the fact that glycerol begins to split off water and to decompose and polymerize at temperatures of the order of 180.degree. C. Crude glycerol is processed in various ways, depending on its salt content. Crude glycerol of low salt content may be directly purified using ion exchangers whereas crude glycerol of high salt content has to be pretreated by distillation to remove the salt. To obtain pure glycerol of the requisite commercial quality, distillation is followed by treatment with ion exchangers, for which purpose the concentrated glycerol has to be rediluted. Both processes are very expensive in terms of apparatus and energy consumption, cannot always be carried out continuously and are accompanied by considerable losses of glycerol. In order to separate glycerol by distillation from higher boiling impurities as well, the mixture has to be additionally subjected to severe thermal stressing which produces further losses of glycerol and more decomposition products.